The proposed work continues our current studies on snail-trematode interaction dynamics using various strains of the host snail Biomphalaria glabrata and various echinostome and schistosome trematode larvae. Trematode-trematode larval competition, predation, growth repression, direct and indirect antagonism, and synergistic stimulation of parasitism have been studied. Our main focus during the past year has been the search for evidence and then the mechanisms of acquired snail immunity to trematode infection. Sensitization and specific resistance has been found, followed by discovery of a highly responsive and rapidly changing amebocyte-producing organ, located in the posterior mantle cavity border and incorporating the adjoining pericardial wall. We have observed rapid encapsulation of trematode larvae in the small heart and in peripheral and visceral tissues, responses we feel are associated with changes in the amebocyte-producing organ. Both innate (genetic) and acquired resistance (following exposure of snails to irradiated echinostome miracidia) show both a sensitization and specific recognition response. Resensitization increases the reactivity of the snail (a type of anamnestic response) and, within experimental limits, evidence for at least a short-term memory has been found. Planned studies will follow these leads and focus on cellular reactions in vitro, hemolymph inoculation studies, and on a variety of experimental infections to determine the response mechanism and to explain the sharp difference in snail response to irradiated and nonirradiated echinostomes and between echinostome and schistosome infections. BIBLIOGRAPHIC REFERENCES: Heyneman, D. (1976) Chapt. 18. Medical Parasitology, in Physicians Handbook (Krupp, Sweet, Jawetz, Biglieri, and Roe, eds). 18th ed. pp 346-395. Lange Medical Publications. Heyneman, D. (1976) Book Review - K.S. Warren, "Schistosomiasis, The Evolution of a Medical Literature. Selected Abstracts and Citations, 1852-1972," Quart. Rev. Biol. 50(2):231-232.